textproduct: Albany

WHAT HAS CHANGED

Excessive Rainfall Outlook was upgraded to a Moderate Risk (level 3 of 4) for the eastern Catskills, Litchfield County, CT, western Massachusetts, the mid-Hudson Valley and central and southern Taconics. We issued a Flood Watch for these areas from 2 PM Tuesday and 8 PM Wednesday in collaboration with neighboring WFOs.

KEY MESSAGES

1) Isolated to scattered showers/thunderstorms today diminish tonight. Areas of rain and thunderstorms capable of heavy downpours redevelop tomorrow afternoon into tomorrow night where there is a moderate (level 3 of 4) to slight risk for excessive rainfall (level 2 of 4) mainly south of I-90 given increased potential for heavy rain to repeatedly impact a given area.

2) Coastal low likely brings periods of rain for tomorrow night through Wednesday night, some of which may be heavy, especially near and south of I-90 and along the east facing slopes of higher terrain area. There is a low to moderate chance for scattered to numerous instances of flash flooding, however threat for river flooding remains very low at this time.

DISCUSSION

KEY MESSAGE 1...

Insolation within a slightly more humid air mass today has generated surface-based instability values to 1000 - 2000 J/Kg. Lingering weak troughing overhead and associated cool pool aloft has resulted in isolated to scattered thunderstorms this afternoon focused mainly in the terrain including the southern Greens, western MA, Litchfield Hills, and parts of the Upper Hudson Valley. Convection will be mainly diurnally driven but a few showers may last through about Midnight. Very weak shear aloft will keep storm slow moving and allow for locally heavy downpours to occur; however, severe weather is not anticipated.

There is increasing confidence for periods of moderate to heavy to develop tomorrow afternoon lasting into tomorrow night. This will occur as our parent shortwave currently in the Midwest rounds the top of the expansive ridge through the Central CONUS and amplifies as it tracks into the eastern Great Lakes tomorrow. Strong upper level diffluence will allow the sfc low that will be in western NY to deepen resulting in a strengthening south-southeasterly low-level jet. This will lead to a strong and deep moisture fetch from the Gulf and western Atlantic with PWATs reaching near 2" resulting in periods of moderate to heavy rain. Initial scattered showers tomorrow morning reaching parts of eastern NY and western New England will transition to more organized areas of moderate to even heavy rain from west to east by 15 - 18 UTC continuing into the overnight hours as the impressive moisture fetch ahead of the low continues to be directed into the Northeast. Mean cloud layer winds look to be oriented nearly parallel to the low-level jet through 09-12 UTC Wednesday and with freezing heights 12 - 13kft and tall skinny CAPE soundings, efficient warm rain processes capable of producing heavy downpours will be at play. The unidirectional flow also suggests training convection or heavy rain repeatedly impacting a given area, raising concerns for flash flooding. The overall dynamics with this event are quite impressive for July with the NAEFS showing height anomalies 3 to 4 standard deviations below normal as the parent trough closes off into Wednesday with integrated water vapor transport 3-4 standard deviations above normal due to the high V-wind component anomalies and PWATs. Latest probabilistic guidance from the HREF shows 40 to 60% chance for 1 inch of rain falling in 3 hours from early afternoon tomorrow through Tuesday night including even 15 to 30% chance for over 2 inches in 3 hours. This indicates multiple hours where high rainfall rates can reach 1 to 2 inches per hour at times with even locally higher amounts possible. The highest probabilities for high rainfall rates reside in the eastern Catskills due to favorable upslope enhancements.

While our recent stretch of dry weather has led to high flash flood guidance (FFG) values including 2-2.5 inches in 1 hour and 2-3 inches in 3 hrs, training convection and heavy rain persisting over a given area for multiple hours can quickly overcome a dry ground resulting in flash flooding. In collaboration with WPC and neighboring WFOs, the heavy rain signal was strong enough for us to issue a Flash Flood Watch for areas mainly south of I-90 from 18 UTC Tues to 00 UTC Thurs with WPC upgrading the Day 2 Excessive Rainfall Outlook to a moderate risk (level 3 of 4) to message potential for numerous instances of flash flooding. There is a slight risk (level 2 of 4) for excessive rainfall for most of the other parts of the area.

KEY MESSAGE 2...

Our parent cyclone becomes a vertically stacked closes low by Wednesday and slows down as it shifts into eastern NY. The moisture plume shifts into New England but continues to wrap around the low throughout the day on Wednesday supporting continued periods of steady rain. Not only will steady rain bands continue within the deformation zone on the north/west of the low but there are signals of a TROWAL developing with weak instability aloft supporting periods of moderate to heavy rain. Given the heavy rain from Tuesday into Tuesday night, soils will likely be saturated and therefore more vulnerable to flooding. At this point, we will likely transition more to areal flooding versus flash flooding as many will be raining for nearly 24 hours at that point but nonetheless, additional flooding concerns continue into Wednesday and our Flood Watch therefore continues through 8PM Wednesday. Despite the long duration of this rain event, the latest MMEFS and Northeast River Forecast Center still indicate no flooding concerns. Wrap around rain continue into Wednesday night and even Thursday as the low will be very slow to exit into New England. Otherwise, temperatures Wednesday and Thursday will remain below normal likely struggling to rise out of the 60s to low 70s given ample cloud coverage and rain.

After nearly three days of rain, total rainfall amounts from Tuesday afternoon through Thursday range 1.5 to 3.5 inches with the highest probabilities for over 3 inches of rain from the NBM in the eastern Catskills, NW CT and east facing slopes of the southwest Adirondacks, southern Greens, Berkshires and Taconics. Our latest forecast shows 3 to 5 inches in these areas with even up to 6 inches in the peaks of the eastern Catskills.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Through 00z Wednesday...Flying conditions are VFR at all terminals as of 7:50 PM EDT. VFR conditions continue through the first half of tonight, but some fog and/or low stratus may form after midnight. Best chance for fog will be at GFL, followed by PSF. IFR or lower vsbys possible with any fog/mist. Some low stratus may also work its way into POU/PSF later tonight, with MVFR to possibly IFR cigs. If there is more low stratus, then fog would be less likely.

Tomorrow morning, some lingering MVFR cigs are possible at POU/PSF after sunrise as low stratus could be slow to mix out if it does indeed form. Otherwise, will see lowering cigs and increasing rain chances through the day tomorrow. Have delayed the onset of rain slightly from previous TAFs, but still expecting widespread heavy rain with MVFR to IFR vsbys and cigs tomorrow afternoon and especially in the evening.

Winds will generally be light and variable tonight, except at ALB where S/SE winds remain around 10 kt through the night. All terminals see S/SE winds increase to 5-10 kt tomorrow morning, further increasing to 10-15 kt with gusts to 20-25 kt possible tomorrow afternoon.

Outlook...

Tuesday Night: High Operational Impact. Definite SHRA...TSRA. Wednesday: High Operational Impact. Definite SHRA...TSRA. Wednesday Night: High Operational Impact. Definite SHRA...TSRA. Thursday: High Operational Impact. Likely SHRA...TSRA. Thursday Night: Low Operational Impact. Slight Chance of SHRA. Friday: Low Operational Impact. Slight Chance of SHRA...TSRA.

ALY WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch from Tuesday afternoon through Wednesday evening for CTZ001-013. NY...Flood Watch from Tuesday afternoon through Wednesday evening for NYZ058>061-063>066. MA...Flood Watch from Tuesday afternoon through Wednesday evening for MAZ001-025. VT...None.